Dielectric support for high frequency coaxial lines

ABSTRACT

A coaxial structure employing a dielectric support positioned and secured within an outer conductor and having a central opening for receiving a short conductive member for joining two sections of coaxially aligned inner conductors whose end faces engage the end faces of the dielectric support member adjacent a central opening provided therein for receiving the short conductive member. The diameter of the short conductive member is less than the diameter of the central opening in the dielectric support to form a hollow, annular-shaped air gap for eliminating the occurrence of damaging peak field strength within the coaxial structure. The side faces of the dielectric support are each provided with annular-shaped compensation grooves wherein the axial thickness of the dielectric support in the region of the compensation grooves is less than the axial length of the annular gap. The size of the annular gap is selected to prevent the occurrence of inadmissable peak field strength.

United States Patent [191 Spinner et al.

Sept. 18, 1973 DIELECTRIC SUPPORT FOR HIGH FREQUENCY COAXIAL LINES 22 Filed: Apr. 23, 1971 211 App]. No.: 136,841

[30] Foreign Application PriorityData Apr. 24, 1970 Germany P 20 20 173.4

[52] 11.8. C1. 333/97 R, 333/33 [51] Int. Cl. 1101p l/00 [58] Field of Search 333/97 R, 33, 34, 333/35, 96; 174/111, 28 R, 99 R [56] References Cited UNITED STATES PATENTS 2,774,944 12/1956 Lintzel 333/97 X 2,623,122 12/1952 Weber et a1. 333/33 X 3,600,709 8/1971 Ditscheid et al 333/34 X FOREIGN PATENTS OR APPLICATIONS 1,064,586 9/1959 Germany 333/97 1,040,631 10/1958 Germany 333/34 27,956 6/1956 Germany 333/97 1,160,520 1/1964 Germany 174/28 1,490,633 7/1969 Germany l 174/28 776,637 6/1957 Great Britain 174/28 OTHER PUBLICATIONS Neubauer et al., Higher Modes in Coaxial RF Lines" Microwave Jr. 6-1969 pp. 62

Ragan, G.L. Microwave Transmission Circuits, McGraw Hill, 1948, pp. 155-170 Primary ExaminerRud o1ph V. Rolinec Assistant ExaminerWm. I-I. Punter Att0rney--Ostr0lenk, Faber, Gerb & Soffen 57] ABSTRACT A coaxial structure employing a dielectric support positioned and secured within an outer conductor and having a central opening for receiving a short conductive member for joining two sections of coaxially aligned inner conductors whose end faces engage the end faces of the dielectric support member adjacent a central opening provided therein for receiving the short conductive member. The diameter of the short conductive member is less than the diameter of the central opening 7 in the dielectric support to form a hollow, annular- 1 Claim, 1 Drawing Figure DIELECTRIC SUPPORT FOR HIGH FREQUENCY COAXIAL LINES The invention relates to a dielectric support for coaxial lines which is supported on a portion of reduced diameter of the inner conductor and is provided with end-face grooves for compensation purposes, the transition to said inner conductor portion from both sides being via a conical surface and an end ring surface bearing on the dielectric support at the end face.

The fixing of the dielectric support to the inner conductor presents considerable difficulties because when air gaps are present between the conducting surface of the inner conductor portion and the dielectric support high field intensities arise in the air spaces present due to the fact that air has a low dielectric constant compared with the dielectric constant of the support. Since however it is in practise extremely difficult or impossible to avoid small air spaces between the inner conductor portion and the dielectric support, the solution adopted has been to provide the internal bore of the dielectric support, consisting for example of ceramic material, with a conductive coating by for example electroplatingmethods, said coating conducting the current over the axial length of the dielectric support. This conductive coating is led outwardly on both sides to an end ring surface at which the support engages axially the end ring surface of the continuing inner conductor.

Admittedly, in this manner air inclusions and the occurrence of high field intensities therein are avoided. However, it can happen that under high current load this coating splits. Moreover, the contacting at the end ring surfaces with the galvanic coating is extremely difficult to effect.

The problem underlying the invention is to avoid the occurrence of damaging peak field strengths without conductive coatings of the dielectric support.

According to the invention this problem is solved in a dielectric support of the type explained at the beginning in that an annular air gap is left at least over the axial extent of the supporting wall of the support between the latter and the inner conductor portion and the mechanical fitting support is effected at the two ends of the dielectric disc in the region of the compensation grooves. p

The invention is based on the knowledge that in the outer region small air inclusions between the supporting parts of the inner conductor and the dielectric support cannot have a disadvantageous effect because in this cross-sectional plane due to the compensation grooves of the support between the outer and inner conductors there is a long air gap and only relatively short gaps in the dielectric of the support, i.e., in the outer portion and at the inner collar.-Due to this long air gap even small air inclusions in this region cannot produce high peak field intensities. In the region of the supporting wall of the support an air gap is deliberately provided and has such a length that the field intensitiesv cannot assume inadmissibly high values. Between the supporting portions and the air gap calculable transitions may be provided, but it sutfices for practical conditions to provide a stepped transition. Thelatter may be provided both in the dielectric support and in the inner conductor portion. For practical reasons the latter alternative is chosenand the inner conductor portion is provided withinthe axial length of the support with two supporting collars on both sides and a portion. of further reduced diameter therebetween. For safety reasons it is advisable for the annular air gap to extend over a greater axial length than the supporting wall of the support.

An example of embodiment of the invention will be described hereinafter with reference to the drawing. The single FIGURE of the drawing shows a section of a support constructed accordingto the invention and inserted in a coaxial conductor.

Inserted between the outer conductor 10 and inner conductor 12 is a dielectric support 13. The latter lies in a groove 14 of the outer conductor 10 and is supported on a reduced-diameter portion of the inner conductor. In the region between the inner and outer conductors the dielectric support comprises for compensation purposes on both sides grooves 15 between which the supporting wall 16 is left with an axial length l. The inner conductor 12 merges via a conical surface 18 into an end ring surface 19 which bears against the dielectric support at the end face and then into a collar 22 whose diameter is matched as accurately as possible to the internal diameter of the inner bore 20 of the dielectric support. In the intermediate portion 24 the inner conductor is further reduced in diameter, resulting in an annular air gap 26 between the inner conductor and the dielectric support.

The axial length of the supporting collars 22 is less than the axial depth of the grooves 15, i.e., the axial length of the annular gap 26 is greater than the axial thickness 1 of the supporting wall 16. This construction results in an acceptable field strength distribution in every cross-sectional plane; any air gaps present in the outer portions do not have a detrimental influence due to the large air gap (small dielectric constant) formed by the grooves 15. In the centre planes, i.e., the crosssectional planes passing through the wall 16, an air gap is connected in series with the dielectric of the dielectric support, said gap having radial dimensions large eough to prevent the occurrence of inadmissible peak field strengths.

What we claim is: I I

l. A coaxial structure comprising a tubular outer conductor;

a disc-shaped dielectric support member having an annular periphery the interior surface of said outer conductor having an annular groove for receiving the periphery of said dielectric support member; a

said dielectric support member having a central opening axially aligned with the longitudinal axis of said outer conductor; v

at least first and second annular shaped inner conductor sections having their longitudinal axes coaxial with the longitudinal axis of said outer conducadjacent ends of said inner conductor sections being positioned on opposite sides of said dielectric support, said inner conductorshaving conical shaped transitions joining the end surfaces of said adjacent ends to the annular surfaces-of said inner conductor sections; p I

each of said end surfaces engaging an adjacent side of said dielectric support member; I

a third inner conductor member having its longitudinal axis coaxial with the longitudinal axis of said outer conductor; said third innerconductor mem her being positioned within the central opening in said dielectric member;

the diameter of said central opening being less than the diameter'of said first and second inner conductor sections;

the diameter of said third inner conductor member being less than the diameter of said central opening whereby the outer surface of said third inner conductor member lies a spaced distance from the surface of said central opening toform a hollow annular shaped air gap;

means for mechanically joining and electrically connecting the ends of said third inner conductor member to the adjacent end surfaces of said first and second inner conductor sections} the faces of said dielectric member each having an annular shaped compensation groove;

said joining means comprising a supporting collar joined to each end surface'and extending a small distance into said central opening;

the axial length of said annular air gap being greater than the axial thickness of the supporting wall of the dielectric support member in the region of said compensation grooves to provide an acceptable field strength distribution, the grooves in said dielectric support member preventing detrimental influences in the surface of the dielectric support member due to the large air gap formed by the annular grooves and the air gap between the dielectric support member and the third inner conductor member being in series with the dielectric support member to prevent the occurrence of inadmissible peak field strengths. 

1. A coaxial structure comprising a tubular outer conductor; a disc-shaped dielectric support member having an annular periphery the interior surface of said outer conductor having an annular groove for receiving the periphery of said dielectric support member; said dielectric support member having a central opening axially aligned with the longitudinal axis of said outer conductor; at least first and second annular shaped inner conductor sections having their longitudinal axes coaxial with the longituDinal axis of said outer conductor; adjacent ends of said inner conductor sections being positioned on opposite sides of said dielectric support, said inner conductors having conical shaped transitions joining the end surfaces of said adjacent ends to the annular surfaces of said inner conductor sections; each of said end surfaces engaging an adjacent side of said dielectric support member; a third inner conductor member having its longitudinal axis coaxial with the longitudinal axis of said outer conductor; said third inner conductor member being positioned within the central opening in said dielectric member; the diameter of said central opening being less than the diameter of said first and second inner conductor sections; the diameter of said third inner conductor member being less than the diameter of said central opening whereby the outer surface of said third inner conductor member lies a spaced distance from the surface of said central opening to form a hollow annular shaped air gap; means for mechanically joining and electrically connecting the ends of said third inner conductor member to the adjacent end surfaces of said first and second inner conductor sections; the faces of said dielectric member each having an annular shaped compensation groove; said joining means comprising a supporting collar joined to each end surface and extending a small distance into said central opening; the axial length of said annular air gap being greater than the axial thickness of the supporting wall of the dielectric support member in the region of said compensation grooves to provide an acceptable field strength distribution, the grooves in said dielectric support member preventing detrimental influences in the surface of the dielectric support member due to the large air gap formed by the annular grooves and the air gap between the dielectric support member and the third inner conductor member being in series with the dielectric support member to prevent the occurrence of inadmissible peak field strengths. 